One minute you're sitting at your desk on solid ground, the next the floor slips out from underneath you and things start to rattle off the shelves. Earthquakes seem to come out of nowhere and can cause widespread devastation.

Building codes and safety videos provide only so much protection.

So governments in some earthquake-prone countries, including Japan and Mexico, institute early warning systems to alert the public to expect potentially hazardous shaking.

This week, California followed suit, adopting a new law that requires the Office of Emergency Services to institute a public early warning system for earthquakes.

But how can you issue a warning for a phenomenon we can't even predict?

We spoke to seismologist Richard Allen, director of the Seismological Laboratory at the University of California, Berkeley, to understand how an earthquake early warning system works and what one might look like for the Golden State.

How can you have an early warning system for something no one can predict?

Basically, when people talk about prediction, they're talking about when an earthquake will occur—when rocks in a fault slip past each other.

Most seismologists say we won't be able to predict this for the foreseeable future.

We're predicting the shaking that comes from when the earthquake ruptures. So basically, an earthquake has already started.

People think of an earthquake as being an instantaneous occurrence. It's not. The energy that radiates out from an earthquake is what causes the shaking that people feel.

P waves come first, and our instruments can detect that. S waves come next, and they carry most of the energy and do most of the damage.

You can estimate how strong the shaking that's carried by the S waves will be, and that's the basis for the early warning.

How far in advance could these warnings be?

We're talking about tens of seconds. The best case on the U.S. West Coast, in California, you could have up to a minute of warning. If you're in Seattle, you could have up to five minutes.

Why is there a timing difference between California and Seattle?

The warning time is a function of your distance from the [earthquake's] epicenter. And the most warning time actually occurs for the bigger earthquakes.

In California, the biggest magnitude you could get is an 8. So you could get up to a minute warning. For the Cascadia Subduction Zone, the biggest magnitude is about a 9, and you could get a five-minute warning.

It's because bigger earthquakes rupture over a much larger area, and it takes time for that earthquake to propagate over the area. And that gives more time for warnings.

If you're right next to the epicenter, you might get no warning at all. The shaking at the epicenter occurs about a second after the P waves. If you're right at the epicenter, you probably will not get a warning.

What can you do with a few seconds of warning?

First is what can people do for themselves? The idea of early warning is that you can get under a sturdy table before the shaking begins. It's about getting mentally ready for the shaking, and then essentially waiting for the earthquake to pass.

The second area is automated response. And that's really about automated systems that can be slowed, stopped, put into a safe mode to reduce damage.

The best example is trains. If you can brake a train so it slows down, you can significantly reduce the chances it will derail during the earthquake. The BART [Bay Area Rapid Transit public transportation system in San Francisco] already has an early warning system.

During rush hour, there are about 75 BART trains running. Each train is carrying about a thousand people. At any given point there are about 45 trains traveling at 70 miles per hour [113 kilometers per hour]. A derailment could result in a lot of injuries.

Another example comes from the Oki computer chip manufacturer in Japan. They had $15 million worth of damage during two moderate earthquakes. They implemented an early warning system that would isolate hazardous chemicals and put chip-manufacturing robots into safe mode.

The next time, they only had $600,000 worth of damage in two similar earthquakes.

All four were in the magnitude 6 range.

What would an early warning system look like in California?

We—meaning the California Integrated Seismic Network, a federation of groups that run the seismic network in the state—have been working on developing a proof of concept of an early warning system.

For the last two years we've had a demonstration system running using the existing seismic network.

It issues an alert to a small group of users: scientists both internal and external to the project, as well as industry partners. BART gets the alert, San Francisco emergency response, Los Angeles city and county emergency response, and then private companies [that] have been testing this system over the last two years.

We're in the process of creating a prototype system that would be a blueprint for California's public early warning system.

We envision that it would build on the prototype system we have. We already have 400 seismic stations all around the state, but they vary in their coverage. Some areas have more than others.

We would need to add some more stations so that we have more coverage over the state.

We envision that the public portion of the system would issue the alerts, but we would need the private sector to distribute them to users.

There could be apps that people could create for smartphones. [Alerts] could also be distributed over the Internet so you could have an app pop up on your desktop.

Can you predict where the shaking will go?

We'll detect where the earthquake starts, the magnitude, and create a prediction map as an estimate of ground shaking. When you get the alert, we can tell you what we expect the shaking to be at your location.

How much would the entire system cost?

The cost of building and operating this system for the first five years is about $80 million in California. If you wanted to do this for the West Coast as a whole, it would be $120 million for the first five years.

To operate it beyond those first five years, it's about $16 million per year.

It's approximately doubling what's currently spent for earthquake monitoring on the West Coast.

How long have Japan and Mexico had their systems?

[Mexico's] was the first public warning system. It went online in 1991, so it's actually been around for a very long time.

Mexico City gets more than 60 seconds worth of warning, because the [usual] source of their earthquakes is about 300 kilometers [186 miles] from the city.

Last year in March there was a magnitude 7.4 that caused a large amount of shaking, fairly serious shaking in Mexico City, and the system issued a warning and people evacuated buildings.

Japan's is by far the most sophisticated early warning system. They turned on their public, nationwide system in 2007.

The really big test was in the magnitude 9 earthquake in 2011. And they successfully issued a warning for that earthquake for the largest nearby city, Sendai. They had about 15 seconds of warning.

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